EXPERIMENTS 


RELATING 

TO  THE  ALLEGED  INFLUENCE  OF  COLOUR 

ON  THE 

RADIATION 

OF 

NON-LUMINOUS  HEAT. 


©  ©*«♦*— 

BY  A.  D.  BACHE, 

Prof,  of  Nat.  Philos,  and  Chem.  University  of  Pennsylvania, 
One  of  the  Secretaries  Am.  Phil.  Soc.  &c. 


INQUIRY 

In  relation  to  the  alleged  Influence  of  Colour  on  the  Radiation  of  Non-lumi - 

nous  Heat . 

BY  A.  D.  BACHE, 

Prof,  of  Nat.  Philos,  and  Chem.  TJniv.  of  Penn. 

[From  the  Journal  of  the  Franklin  Institute  for  November,  1835.] 


In  the  following  essay  I  propose  to  submit  a  few  remarks  upon  a  pa¬ 
per  by  Doct.  Stark  of  Edinburgh,  first  published  in  the  Transactions  of 
the  Royal  Society  of  London,  for  1 833,  together  with  an  experimental 
inquiry  into  the  alleged  influence  of  colour  on  the  radiation  and  absorp¬ 
tion  of  non-luminous  heat. 

The  experiments  were  commenced  soon  after  the  paper  referred  to, 
reached  this  country,  and  in  them  was  adopted  what  seemed  to  me  the 
less  exceptionable  of  two  methods  used  by  Doctor  Stark,  which  actually 
bear  upon  the  question  of  the  radiation  of  non-luminous  heat.  It  was  my 
intention  to  examine  the  matter  more  fully  than  had  been  done  by  Dr. 
Stark,  and  to  procure  a  more  satisfactory  induction  by  experimenting 
on  a  considerable  variety  of  substances.  In  this  I  had  the  kind  assist¬ 
ance  of  my  colleague,  Prof.  Courtenay* 

While  these  experiments  were  in  progress,  the  remarks  of  the  Rev. 
Professor  Powell,  of  Oxford,  on  the  paper  of  Doctor  Stark,  appeared 
in  the  Edinburgh  New  Philosophical  Journal.  They  confirmed  me  en¬ 
tirely  in  the  view  of  the  inapplicability  of  most  of  the  experiments  made 
by  Doct.  Stark,  to  the  determination  of  the  question  of  the  influence  of 
colour  on  the  radiation  or  the  absorption  of  heat.  Of  this  class  were  the 
absorption  of  heat,  radiant  heat  being  understood,  as  tested  by  the  in¬ 
verse  of  Count  Rumford’s  method  for  comparing  the  conducting  powers 
of  substances  used  for  clothing;  also  as  tested  by  the  effect  of  the  heat 
from  the  flame  of  an  argand  gas  burner,  thrown  by  a  mirror  upon  the 
bulb  of  an  air  thermometer,  which  was  variously  coated.  Of  the  same 
class  were  the  experiments  on  radiation,  as  tested  by  the  method  used  by 
Count  Rumford,  above  referred  to;  the  enveloping  materials  on  the 
inner  thermometer  being  wools  of  different  colours,  and  coloured  wheat- 
en  paste. 

Not  included  in  this  class  were  the  methods  of  ascertaining  the  rate  of 
cooling  of  a  thermometer  of  which  the  bulb  was  coated  with  different 
pigments,  and  of  a  glass  globe  filled  with  warm  water  and  variously  coat¬ 
ed.  I  give  the  preference  to  a  modification  of  this  latter  method  from 
the  greater  extent  of  radiating  surface  which  may,  without  inconvenience, 
be  commanded  by  it.  The  glass  globe  used  by  Dr.  Stark,  was  one  inch 
and  a  quarter  in  diameter;  it  was  coated  at  different  times  with  Prus¬ 
sian  blue,  red  lead,  and  white  lead,  and  in  a  room  at  50°  Fah.,  the  fall  of 
temperature  from  120°,  through  25  degrees,  was  with  the  coatings  re¬ 
spectively,  in  seventeen  minutes,  eighteen  minutes,  and  nineteen  minutes. 

I  am  constrained  to  differ  from  Professor  Powell  in  his  remarks  upon 
the  method  just  referred  to,  and,  with  great  deference  to  so  high  autho¬ 
rity,  would  state  why  I  consider  them  inconclusive.  Professor  Powell 
deems  it  necessary,  or  at  least  highly  important  to  the  determination  of 
the  question,  that  the  radiating  coatings  of  the  globe  should  be  equalized 

1 


4 


in  respect  to  thickness,  conducting  power,  density,  &c.,  and  refers  to 
the  experiments  of  Prof.  Leslie,  in  which  equal  quantities  of  different 
radiating  substances  were  dissolved  and  spread  upon  a  surface,  for  com¬ 
parison.  That  equal  thicknesses  of  substances  possessing  different  ra¬ 
diating  powers,  should  be  compared  together,  seems  to  me  to  be  disprov¬ 
ed  by  the  law  established  by  Sir  John  Leslie’s  own  experiments,  name¬ 
ly,  that  radiation  takes  place  not  only  from  the  surface,  but  in  a  thick¬ 
ness  which  is  appreciable  in  good  radiators.  This  thickness  not  being 
the  same  for  different  substances.  Thus  when  different  coatings  of 
jelly  were  applied,  in  succession,  upon  one  of  the  sides  of  the  cube  in 
Professor  Leslie’s  experiments,  the  radiation  increased  with  the  thick¬ 
ness,  up  to  a  certain  point.  The  effect  of  conducting  power  appears,  by 
this  same  experiment,  to  be  so  small  that  an  increase  of  the  thickness  in 
the  bad  conductor  was  actually  more  than  compensated  for  by  the  in¬ 
creased  radiating  power.  The  influence  of  density  on  conducting  pow¬ 
er  is  well  known,  but  the  effect  of  either  as  controlling  the  radiating  pow¬ 
er  of  a  substance,  or  as  modifying  it,  is,  I  apprehend,  yet  to  be  appre¬ 
ciated.  If  these  views  be  correct,  and  they  are,  I  believe,  founded  upon 
the  authorities  so  ably  illustrated  by  Professor  Powell  in  his  report  on 
radiant  heat,  to  the  British  Association,  the  radiating  powers  of  substan¬ 
ces  would  not  be  rightly  compared  by  equalizing  their  thicknesses  upon 
I  a  given  surface,  nor  by  equalizing  their  weights;  but  by  ascertaining, 
for  each  substance,  that  thickness  beyond  which  radiation  does  not  take 
place.  This  will  be  placed  in  a  clearer  point  of  view  in  the  sequel. 

I  do  not,  however,  consider  the  question  at  issue  as  the  less  difficult  to 
determine;  “no  substance  can  be  made  to  assume  different  colours  with¬ 
out  at  the  same  time  changing  its  internal  structure,”*  and  I  believe  with 
Professor  Powell,  that  “a  very  extensive  induction  is  perhaps  the  only 
means  open  to  us  of  ascertaining  this,  (the  circumstances  and  properties 
[wherein  the  coatings  differ)  considering  how  totally  ignorant  we  are  of 
the  peculiarities  on  which  their  colour  depends.” 

This  very  extensive  induction  I  do  not  pretend  to  have  made,  but  I 
think  to  have  multiplied  experiments  so  much  beyond  the  number  made 
by  Dr.  Stark,  as  to  be  able  to  show  that  the  supposed  influence  of  colour 
on  the  absorption  and  radiation  of  heat  remains  yet  to  be  demonstrated, 
and  thus  to  prevent  the  admission  as  proved  of  what  is  more  than  doubtful. 

The  principal  object  was  to  select  a  considerable  variety  of  pigments 
of  the  same  colour  differing  chemically,  and  of  different  colours  chemi¬ 
cally  allied,  and,  as  subsidiary,  to  ascertain  the  effect  of  changes  of 
colour,  produced  by  chemical  means,  on  different  substances,  and  the 
effect  of  the  material  used  to  apply  the  pigment  to  the  radiating  body. 

Several  tin  cylinders  were  procured,  two  inches  high,  and  an  inch  and  a 
half  in  diameter,  closed  at  the  bottom,  and  having  fitted  to  the  top  a  slight¬ 
ly  conical  tube,  to  receive  a  perforated  cork,  through  which  to  pass  the 
stem  of  a  thermometer.  One  of  these  vessels  having  been  selected,  was 
coated  in  successive  layers  with  a  pigment.  Water  which  was  boiling 
in  a  porcelain  capsule  was  then  poured  into  the  cylinder,  which  was 
suspended  by  means  of  two  lateral  hooks  to  cords  attached  to  the  cano¬ 
py  covering  the  lecture  table.  A  thermometer  introduced  through  a 
cork  had  its  bulb  nearly  in  the  middle  of  the  axis  of  the  cylinder,  and 
the  thermometer  by  displacing  part  of  the  water  assured  that  the  quan- 


*  Prof.  Leslie’s  Essay  on  Heat. 


5 


tity  contained  was  the  same  in  each  case.  A  temperature  was  selected 
for  beginning  the  experiments,  sufficiently  below  that  which  the  intro¬ 
duction  of  boiling  water  produced,  to  permit  the  rate  of  cooling  to  have 
become  uniform,  and  one  for  ending  which  was  high  enough  to  prevent 
uncertainty  from  the  slowness  of  the  fall  of  temperature.  The  instant  of 
the  arrival  of  the  mercurial  column  at  any  degree  on  the  scale,  and  of 
its  leaving  the  same,  was  noted,  and  a  mean  taken  for  the  time  of  being 
at  that  temperature;  a  precaution  which  though  superfluous  in  such 
experiments  as  these,  will,  I  am  persuaded,  be  found  of  importance 
where  minute  accuracy  is  desired  in  investigating  the  motion  of  heat. 
One  of  us  observed  the  thermometer,  the  other  noted  the  time  by  a 
pocket  chronometer. 

The  time  of  cooling  of  the  cylinder  coated  with  colouring  matter 
having  been  ascertained,  an  additional  layer  of  the  same  substance  was 
put  upon  it  and  the  cooling  again  observed.  The  time  of  cooling  di¬ 
minished,  of  course,  until  that  thickness  was  obtained  beneath  which 
no  radiation  takes  place,  the  time  then  slowly  increased  with  each  ad¬ 
ditional  coat,  the  conducting  power  entering  as  an  appreciable  element 
into  the  rate  of  cooling.  To  show  the  decided  nature  of  the  results,  I 
subjoin  an  account  of  one  series  towards  the  beginning  of  our  experi¬ 
ments,  when  a  want  of  experience  rendered  us  cautious  in  applying  the 
successive  coatings,  lest  we  should  pass  the  thickness  of  determinate 
radiation.  The  necessity  for  thus  feeling  our  way,  rendered  the  labour 
of  the  experiments  very  considerable. 

Cylinder  coated  with  Prussian  blue: 

Time  of  cooling  from  180°  to  140°  Fah. 

1.  Thick  coating,  -  -  101  lj 

2.  ditto  added,  -  965 

3.  Additional  coat,  -  -  910| 


seconds. 


4. 

do. 

do. 

- 

829J 

5. 

do. 

do. 

- 

805 

6. 

do. 

do. 

- 

824 

Another  series,  in  a  further  advanced  stage  of  our  experiments  is 
subjoined: 


Cylinder  coated  with  Litmus  blue: 
Time  of  cooling  from  180°  to  1403  Fah. 

1.  First  thick  coating, 

2.  Additional  coat, 

3.  do.  do. 

4.  do.  do. 


985  seconds. 

855 

82  7\ 

8341 


Besides  the  necessity  of  making  several  experiments  to  obtain  a  sin¬ 
gle  result,  it  sometimes  occurred  that  particular  results  required  to  be 
repeated  for  verification,  when  apparent  discrepancies  occurred;  this 
was  done  to  ascertain  if  they  were  real  or  not. 

As  it  was  obvious  that  the  experiments  must  necessarily  extend 
through  a  considerable  time,  during  which  the  circumstances  attending 
the  cooling  of  the  cylinders  could  not  be  expected  to  remain  uniform, 
a  standard  for  comparison  was  provided,  in  a  cylinder  of  which  the 
coating  was  not  changed,  and  which  was  observed  in  regular  turn  with 
the  other  cylinders.  At  first  a  vessel  without  coating  was  used  for  this 
purpose,  but  as  it  was  found  liable  to  tarnish,  it  was  substituted  by  a 
cylinder  having  a  coating  of  aurum  musivum,  which  was  one  of  the 


6 


smoothest  and  most  uniform  of  the  coloured  coatings  used.  The 
numbers  obtained  on  the  different  days  from  a  mean  of  the  trials  made 
of  the  cooling  of  the  standard  cylinder,  were  applied  to  compare  the 
results  of  one  day  with  those  of  another.  This  assumes  that  the  times 
of  cooling  of  the  different  vessels  would  be  affected  proportionately  by 
a  given  change  in  the  circumstances  of  the  experiment.  The  inability 
to  preserve  the  circumstances  constant  is  the  real  objection  to  this 
method,  and  one  which  most  affects  the  certainty  of  the  results.* 

The  following  example  shows  the  application  of  this  method.  The 
observed  times  of  cooling  of  the  standard  cylinder,  from  180  to  140° 
in  two  experiments  on  the  31st  of  October,  were  969}  and  968}  seconds, 
mean  969.  Three  experiments  on  the  first  of  November,  gave  898,  892, 
and  893}  seconds,  mean  894}. 

Cylinder,  number  four,  coated  with  cochineal,  (crimson,)  gave  for  the 
time  of  cooling  from  180°  to  140°,  on  the  1st  of  November,  848}.  To 
compare  this  with  a  result  obtained  with  the  same  cylinder,  on  the  31st 
of  October,  we  have  894}  :  969  ::  848}  :  x,  the  equivalent  number  for 
October  31st,  916.3  seconds. 

The  results  obtained  with  the  same  cylinder,  on  different  occasions  of 
experiment,  having  been  thus  rendered  comparable,  the  comparison  of 
experiments  with  different  cylinders  was  effected  by  determining  the 
time  of  cooling  with  the  same  coating  upon  different  cylinders.  Thus, 
numbers  one  and  two  having  been  coated  with  carbonate  of  lead,  and 
their  times  of  cooling  through  forty  degrees  having  been  ascertained,  all 
the  results  with  the  various  other  coatings  applied  to  these  cylinders 
were  comparable. 

The  numbers  thus  obtained  will  not  be  strictly  proportional  to  the 
radiating  power  of  the  substance  used,  for  the  whole  surface  of  the  cy¬ 
linders,  including  the  ends,  was  not  coated,  and  the  contact  of  the  air, 
and  its  consequent  circulation,  exert  a  most  important  influence  on  the 
rate  of  cooling.  This  latter  element  has  been  shown  by  the  experiments 
of  Petit  and  Dulong,  to  be  independent  of  the  nature  of  the  surface,  and 
as  the  amount  of  uncoated  surface  remains  constant,  the  greater  effect 
of  radiation  will  appear  by  the  more  rapid  rate  of  cooling,  and  the  less 
by  the  less  rapid  rate. 

I  proceed  now  to  examine  the  degree  of  approximation  which  maybe 
expected  from  the  results  of  the  experiments. 

First,  a  comparison  of  different  observations  on  the  same  day,  under 
the  same  circumstances  of  the  cylinders,  and  nearly  or  quite  the  same  as 
to  the  temperature  of  the  room,  will  show  how  far  accuracy  is  possible 
under  the  most  favourable  suppositions.  The  following  table  presents 
the  results  of  this  kind  obtained  during  the  entire  series  of  experiments, 
with  the  ratios  of  the  times  of  cooling. 

*  If  the  circumstances  could  be  retained  the  same,  three  observations  of  the  tem¬ 
perature  at  equal  known  intervals,  would  give  a  numerical  expression  for  the  radiating 
power  of  the  coating. 


1 


Nature  of  Coating. 

Time 

in 

CPP*C 

Ratio. 

Nature  of  Coating. 

Time 

in 

QPP*Q 

Ratio. 

Cylinder  No.  3. 

OC  O • 

Cylinder  No.  1. 

No  coating. 

1281$ 

1300 

1.000 

1.014 

Sulphuret  of  Antimony. 

849J 

972f 

1.000 

1.145 

Chalk. 

909* 

939! 

1.000 

1.034 

do.  additional. 

Coating  on  another  occ’n. 

871! 

878! 

1.000 

1.008 

Prussian  blue. 

909! 

932! 

1.000 

1.025 

Red  lead. 

886! 

894! 

1.000 

1.009 

Litmus  blue. 

920! 

956 

1.000 

1.038 

do.  blackened 
by  sulphuretted  hydrogen 

911! 

924! 

1.000 

1.014 

Cylinder  No.  5. 

Cylinder  No.  4. 

Aurum  Musivum. 

892 

1.000 

Gamboge 

932 

1.000 

893! 

1.001 

942! 

1.011 

898 

I  007 

938! 

1.000 

do.  on  another 

937! 

1.000 

Chromate  of  lead. 

954! 

1.017 

occasion. 

959 

1.023 

845 

1.000 

943! 

1.000 

Vermilion. 

850 

1.006 

do. 

957 

1.014 

740! 

1.000 

818 

1.000 

Sulphate  of  Baryta. 

778 

1.051 

do. 

820! 

1.003 

— 

850 

1.000 

Cylinder  No.  1. 

do. 

860 

1  012 

897 

1.055 

No  coating. 

1396! 

1.000 

— 

1425! 

1.020 

851 

1.000 

1445! 

1.035 

do. 

872! 

1.025 

1313! 

1.000 

do.  another  occasion. 

1315! 

1.002 

do. 

1303 

1.000 

1320 

1.013 

In  the  foregoing  table,  ten  of  the  ratios  are  about  l.ol  to  l,six  1.02  to 
1,  three  1,03  to  1,  one  1.04  to  1,  and  two  1.05  to  1:  it  is,  therefore,  fair  to 
infer  that  the  single  ratio  of  1.14  to  1  results  from  an  error  of  record,  or 
observation,  and  the  table  fully  shows,  that,  under  the  same  circumstances 9 
the  results  could  readily  he  reproduced  within  about  two  per  cent. 

Second.  The  correction  for  the  altered  circumstances  of  temperature 
.  V16,.!?01*1’  &c:>™y  ke  tested  by  comparing  the  experiments  made 
with  different  cylinders,  having  the  same  coatings,  on  different  days.  In 
the  annexed  table  is  given  the  various  results  of  this  kind  furnished 
throughout  the  series  of  experiments.  The  date  is  given  in  the  left  hand 
column,  and  applies  to  all  the  results  on  the  same  horizontal  line  with  it. 
A  comparison  of  the  numbers  in  the  columns  marked  ratio,  and  on  the 
same  horizontal  lines,  will  show  how  far  the  same  reduction  to  a  stand¬ 
ard  would  have  been  given  by  different  cylinders;  in  other  words,  how 


8 


far  the  influence  of  currents  of  air,  local  temperature,  and  radiation  from 
or  to  adjacent  bodies,  might  have  interfered  with  the  particular  results. 


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Of  the  ratios  thus  brought  into  comparison,  it  will  be  found  that,  in 
one  case,  the  results  are  identical;  in  four  others,  that  they  differ  one  per 
cent.;  in  two  others,  two  per  cent.;  in  four  others,  three;  in  one,  four;  in 


9 

three,  five;  in  two,  seven;  and  in  one,  ten  per  cent.;  omitting  this  latter, 
the  accordance  is  much  less  satisfactory  than  was  shown  by  the  former 
table,  and  the  average  amount  of  error  is  nearly  four  per  cent. 

Having  now  shown  the  probable  limits  of  accuracy  in  the  experiments, 
I  proceed  to  compare  the  reduced  times  of  cooling  of  the  same  cylinders 
with  different  coatings.  In  the  table  will  be  given  the  observed  time  of 
cooling  through  forty  degrees,  and  the  time  of  cooling  of  the  standard, 
from  whence  the  reduced  times  are  deduced.  As  the  colours  of  the  sub¬ 
stances  were  not  in  all  cases  what  would  be  expected,  the  colour  is  de¬ 
signated  in  a  separate  column. 


Cylinder  No.  1,  variously  coated. 


Nature  of  Coating. 

Colour. 

Date. 

|  Observed  time 
of 

f  cooling. 

Time  of  cooling 

of 

standard. 

Reduced  time 

of 

cooling. 

Remarks. 

sec’ds. 

st  c’ds. 

sec’ds. 

Carbonate  of  lead. 

White. 

Oct.  24 

864 

1014 

864 

Smooth. 

Vermillion. 

Golden  Sulphu-T 

Red. 

Brown, 

25 

806 

937 

872 

Smooth,  with  mi¬ 
nute  cracks. 

ret  of  Antimo-  < 
ny  C 

nearly 

black. 

^31 

868.5 

969 

909 

Rough, peels  easily 

Red  Oxide  of  lead. 

Orange. 

Nov.  6 

890  5 

948.2 

952 

Smooth. 

do.  additional  coat. 

11 

932.7 

950.2 

995 

For  comparison 
with  following. 

Do.  blackened  by 

hydro  sulphate 

Brown. 

917.8 

U 

966 

Red  shows  thro*. 

ot'  potassa. 

Plumbago. 

Black. 

17 

787 

819.2 

974 

Uniform,  but  not 
glossy. 

Gamboge. 

Olive. 

20 

808.7 

816. 

1005 

Smooth,  but  in 
streaks. 

The  radiating  power  being  greater,  as  the  time  of  cooling  is  less,  we 
have  the  order  of  radiating  power  of  the  different  coloured  substances,  as 
follows:  white,  red, brown,  orange,  black,  green;  omitting  in  this  enu¬ 
meration  the  blackened  surface  of  the  red  oxide  of  lead,  which  had  pass¬ 
ed  in  thickness  the  maximum  radiating  thickness,  and  is  only  compara¬ 
ble  with  the  result  which  precedes  it.  The  change  effected  by  altering 
the  surface  to  sulphuret  of  lead,  (black,  or  rather  brown,)  appears  to  in¬ 
crease  the  radiating  power  in  the  ratio  of  1.03  to  1,  which  is,  however, 
within  the  average  of  error. 

The  following  results,  given  in  order  of  time,  and  reduced  by  the 
standard,  were  obtained  with  cylinder  No.  2. 


2 


10 


Nature  of  coating. 

Colour. 

Date. 

Observed  time  i 

of 

cooling. 

Time  of  coolingl 

of 

standard. 

Reduced  time 

of  cooling. 

Remarks. 

sec’ds. 

sec’ds. 

sec’s 

Ammoniacal  sul- 

Blueish 

phate  of  copper. 

green. 

Nov.  6 

808.5 

948.2 

856 

Streaked  and  peels 

off,  rough. 

Indigo. 

Blue. 

11 

928. 

950.2 

990 

Very  smooth. 

Carbonate  of  lead. 

White. 

14 

883.2 

956. 

987 

Smooth. 

do. 

15 

910 

856.5 

982 

For  comparison 

do.  blackened  b) 

with  following. 

hydro  sulphate 

of  potassa. 

Black. 

15 

874 

944 

Per  oxide  of  Man- 

ganese. 

Dark  brown 

18 

747 

869 

872 

Uniform  but  not 

smooth. 

The  variety  of  colour  is  here  small;  the  radiating  powers  rank,  blueish 
green,  dark  brown,  white, blue;  omitting  the  second  experiment  with  the 
carbonate  of  lead,  which  is  only  comparable  with  the  one  in  which  the 
surface  was  blackened  by  hydro-sulphate  of  potassa.  Comparing  these 
two  results,  the  change  of  surface  appears  to  have  increased  the  radia¬ 
ting  power  in  the  ratio  of  1.04  to  1.  The  coatings  applied  to  cylinder 
No.  3  were  more  varied  than  those  of  either  of  the  foregoing. 

Cylinder  No.  3. 


Nature  of  coating. 

Colour. 

Date. 

Observed  tune 
of 

cooling. 

Time  of  cooling 
of 

standard. 

Reduced  time 
of  cooling. 

Remarks. 

sec’ds. 

sec’ds. 

sec’s 

Carb.  of  magnesia. 

Yellowish 

white. 

» 

Oct.  11 

859.5 

862 

1011 

Rough,  in  specks 
projecting, 
do. 

Smooth  and  some¬ 
what  shining. 
Rough. 

Carbonate  of  lime, 
(chalk) 

Carbonate  of  lead. 

Prussian  blue. 

White. 

White. 

Blue. 

25 

879 

877 

805 

937 

1034 

1032 

871 

Litmus. 

Blue. 

31 

831 

969 

870 

Not  uniform. 

Bichromate  potassa 

Reddish 

brown. 

Nov.  1 

854 

894.5 

986 

Streaked  and  not 

Alkanet. 

Crimson. 

11 

926.7 

950 

989 

smooth. 

Uniform. 

Do.  rendered  blue 
by  potassa. 

India  ink. 

Blue. 

Black. 

17 

9382 

776 

819 

1001 

959 

Not  smooth. 

do. 

18 

836 

869 

976 

More  uniform, 

Carbonate  of  lead 
in  oil  of  lavender. 

White. 

21 

843.5 

862 

992 

(mean  967) 

Uniform,  but  not 

Do.  blackened  by 
hydro  sulphate 
of  potassa. 

Black. 

850 

1000 

glossy  on  surface. 

11 


The  effect  of  changing  the  crimson  of  alkanet  to  a  blue,  was,  appa¬ 
rently,  to  decrease  its  radiating  power  about  one  per  cent., or  the  change 
of  colour  in  reality  did  not  alter  the  power.  The  carbonate  of  lead  lost 
also  slightly,  or  rather  was  not  affected,  by  the  change,  not  only  of  its 
surface,  but  of  a  considerable  part  of  its  mass,  for  the  oil  of  lavender 
having  evaporated,  the  hvdro-sulphate  of  potassa  penetrated  the  coating. 
The  substance  by  means  of  which  the  coating  was  applied,  seems  not  to 
have  sensibly  affected  the  radiating  power;  the  carbonate  of  lead,  applied 
with  gum  differing  in  radiating  power  but  four  per  cent,  from  that  ap¬ 
plied  with  oil  of  lavender. 

The  colours  rank  from  the  foregoing  table,  blue,  two  varieties;  black, 
brown,  crimson,  white,  black,  blue,  white,  three  varieties.  There  is  no 
certainty  that  the  litmus  and  the  alkanet  changed  to  blue  by  potassa,  were 
originally  the  same  in  colour.  The  surfaces  were  very  different  in  regard 
to  uniformity  and  smoothness  ;  the  alkanet  was  perfectly  uniform,  but 
not  at  all  glistening;  it  may  be  described  as  of  a  uniformly  minute  rough¬ 
ness.  In  this  table,  we  have  the  greater  number  of  whites  at  the  bottom 
of  the  scale  of  radiation,  and  of  blues  and  blacks  at  the  top;  but  this  is  all 
that  can  be  said,  for  a  white,  a  black,  and  a  blue,  are  in  close  proximity 
near  the  middle  of  the  scale. 

The  results,  with  cylinders  Nos.  4  and  5,  were  few  in  number.  They 
are  subjoined. 


Cylinder ,  No.  4. 

I 

Cochineal, 

Crimson, 

Nov.  1 

848.5 

894.5 

962 

Not  uniform. 

Chromate  of  lead, 

Bi  sulph’t.  of  mercury, 

Yellow, 

6 

931.7 

948.5 

996 

Very  smooth  and  uni¬ 
form. 

(vermilion) 

Red, 

11 

843.7 

950.2 

888 

Uniform  and  smooth. 

Sulphate  of  baryta. 

White, 

15 

759.2 

865.2 

889 

Rough. 

Ditto, 

21 

829 

861.7 

9  75 

Smooth,  freshly  preci¬ 
pitated. 

Cylinder ,  No.  5. 

Gamboge, 

Olive, 

Oct.  29 

845.5 

934 

917 

Smooth. 

Bi-sulphuret  of  tin,  (au- 

rum  musivum,) 

Yellow, 

31 

969 

969 

1014 

Very  even. 

The  order  from  cylinder  No.  4,  is  red,  white,  crimson,  white,  yellow; 
the  influence  of  the  roughness  of  surface  is  here  plainly  shown,  by  which 
the  place  of  the  white  material,  sulphate  of  baryta,  is  entirely  changed; 
this  is  a  quality  difficult  to  appreciate,  and  yet  here  we  find  it  exceeding 
in  influence  any  other  property  of  the  coating. 

A  review  of  these  results  will  show  that  we  have  been  able  to  establish, 
among  the  separate  series,  no  order  of  colour;  we  have  the  different  or¬ 
ders  as  follows. 


From  No.  1. 

No.  2. 

No.  3. 

No.  4. 

White, 

Green, 

Blue, 

Red, 

Red, 

Brown, 

Black, 

White, 

Brown, 

White, 

Brown, 

Crimson, 

Orange, 

Blue, 

Crimson, 

White, 

Black, 

White  to  black,  an 

White, 

Yellow. 

Green, 

increase  of  4  per 

Black, 

No.  5. 

White  to  black,  an 

cent,  in  radiating 

White, 

No  effect  from  chan¬ 

Green, 

increase  of  3  per 

power. 

Yellow. 

cent,  in  radiat¬ 

ging  white  to  bl’k, 

ing  power. 

or  purple  to  blue. 

12 


A  more  satisfactory  comparison,  in  respect  to  the  number  of  substances 
employed,  will  be  had  by  using  the  means,  heretofore  described,  for  com¬ 
paring  together  the  results  obtained  with  different  cylinders.  For  ex¬ 
ample,  Nos.  1,  2,  and  3,  were  each  coated  with  carbonate  of  lead,  and 
through  the  numbers  given  by  these. coatings,  those  found  for  the  other 
coatings  can  be  compared ;  Nos.  1  and  4  were  coated  with  vermillion,  and 
Nos.  1  and  5  with  gamboge. 

The  following  table  presents  the  comparison,  the  substances  being  ar¬ 
ranged  in  the  order  of  their  radiating  powers. 


Number. 

Nature  of  coating. 

Colour. 

Number  of 
Cylinders. 

Date. 

Time 

of 

Cooling. 

Remarks  on 
Surface. 

sec’ds. 

1 

Litmus  blue, 

Blue 

No.3 

Oct.  31 

728 

2 

Prussian  blue, 

Blue 

o 

O 

25 

729 

Rough. 

3 

Ammoniacal  Sul- 

phate  of  copper. 

Greenish  blue 

2 

Nov.  6 

789 

Rough. 

4 

Per-oxide  of  man- 

ganese, 

Brownish  bl’k 

2 

18 

804 

Not  shining,  but 

uniform. 

.5 

India  ink, 

Black 

n 

O 

17 

804 

Nfot  smooth. 

6 

Bi-chromate  of  po- 

tassa, 

Brown 

3 

1 

810 

Streaked,  streaks 

smooth. 

7 

India  ink, 

Black 

n 

O 

18 

817 

Smooth. 

8 

Alkanet, 

Crimson 

3 

11 

828 

Not  shining,  but 

uniform. 

9 

Carbonate  of  lead 

in  oil  of  lavender 

White 

o 

O 

21 

830 

Smooth,  not  shin’g 

10 

Sulphuret  of  lead, 

Black 

o 

21 

837 

11 

Alkanet  blue, 

Blue 

3 

11 

838 

12 

Carbonate  of  mag- 

nesia, 

White 

o 

O 

Oct.  13 

846 

Rough. 

13 

Carbonate  of  lead 

in  gum. 

White 

1 

24 

864 

Smooth. 

14 

Carbonate  of  lime, 

Dingy  white 

n 

O 

11 

865 

Medium. 

15 

Vermilion, 

Red 

1 

25 

872 

Smooth. 

16 

Sulphate  of  baryta, 

White 

4 

Nov.  15 

873 

Rough,  blueish 

white. 

17 

Golden  sulphuret 

of  antimony, 

Brown 

1 

Oct.  31 

909 

Smooth,  in  streaks. 

18 

Indigo, 

Blue 

2 

Nov.  11 

912 

Smooth. 

19 

Cochineal, 

Crimson 

4 

1 

944 

Smooth. 

20 

Red  lead, 

Orange 

1 

6 

952 

Smooth. 

21 

Sulphate  of  baryta, 

White 

4 

21 

957 

Medium. 

22 

Plumbago, 

Black 

1 

17 

974 

Not  shining,  but 

uniform. 

23 

Chromate  of  lead, 

Yellow 

4 

6 

977 

Smooth. 

24 

Gamboge, 

Olive  green 

1 

20 

1005 

Smooth,  in  streaks 

25 

Bi-sulphuret  of  tin, 

Yellow 

5 

Oct.  31 

1085 

Smooth. 

The  results  thus  exhibited  are  decidedly  unfavourable  to  the  specific 
effect  of  colour  in  determining  the  radiating  powers  of  bodies.  Blue  is 
above  black  at  the  beginning  of  the  table,  and  occurs  again  in  the  eigh¬ 
teenth  place.  Although  the  first  seven  numbers  are  blue,  or  black,  the 
ninth,  tenth,  eleventh,  and  twelfth,  are  white,  black,  blue,  and  white, 


13 


respectively.  Red  occupies  the  eighth  and  nineteenth  places,  and  then 
an  intermediate  one,  namely,  the  fifteenth.  White  is  in  the  greater  num¬ 
ber  of  cases  in  the  middle  part  of  the  table,  ranging  close  to  black. 

The  alleged  advantages  of  dark  clothing  during  cold  weather,  thus 
seems  to  have  been  too  hastily  inferred;  and  it  appears  that,  provided  the 
person  is  not  exposed  to  the  sun,  the  particular  colour  of  the  clothing  is 
not  of  real  importance. 

If  colour  is  not  a  determining  quality,  neither  does  roughness  appear 
to  be  so,  for  though  generally  the  smooth  surfaces  are  lower  on  the  list, 
this  is  not  universal.  The  rough  sulphate  of  baryta  is  lower  on  the  list 
than  the  smooth  carbonate  of  lead.  Plumbago  occupies  a  low  place,  and 
India  ink  a  comparatively  high  one. 

The  best  radiators  do  not  appear  to  belong  to  any  particular  class  of 
bodies;  litmus  blue  and  Prussian  blue  ate  side  by  side,  while  sulphuret 
of  lead,  and  the  bi-sulphuret  of  tin  are  fifteen  numbers  apart. 

If  the  results  be  admitted  as  decisive  of  the  radiating  powers  of  the 
bodies  used  they  show  that  each  substance  has  a  specific  power  not  de¬ 
pending  upon  chemical  composition,  nor  upon  colour.  I  do  not  claim  to 
found  such  a  conclusion  upon  the  experiments;  their  object  has  been 
before  stated,  and  if  they  shall  prevent  the  introduction  of  an  inference 
from  an  imperfect  induction,  as  a  law  of  science,  the  labour  bestowed 
upon  them  will  be  amply  recompensed.* 

*  The  scientific  reader  need  not  be  reminded  that  these  remarks  do  not  bear  upon 
tlie  radiation  or  absorption  of  heat  accompanying  light. 


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